What you will need:
1) Masking Tape - to create the initial "mold" from your body.
2) Glue Gun and Glue Stick - for adhesive and effects
3) Plastic straws - for machinery effects
4) Spirit Gum - for attaching effects on skin
5) Elastomeric Paint Red Color - its the special kind of paint used on roofs. Its very thick since it serves to plug small holes in roofs, not mention prevent rust.
6) Rubber Sheet - the main material for metal and machinery effects
7) Black and Silver spray paint - for color
8) Wax Paper - to be used as mat for the effects

The rubber sheets should have at least been sprayed silver, but try to spray them in one direction and not entirely painted, as to simulate gradients and shadows.

Then cut the rubber sheet to the shapes you would want to have your machinery exposed. In this example i want a small patch of metal to show off my scar.

If you noticed, i have also made a cut across the small sheet. The cut represents the metal panels a cyborg may have on his metallic endoskeleton. For better effect, i separated the cut and placed one piece over the other for more pronounced effect.

Prepare as many shapes and varieties as you want, but keep in mind those very shapes for the next important step.

02: Making the Mold
a) Lay down a mat of Wax Paper on your work table.
b) With a Cotton Bud (or Q-Stick), dip one end on Elastomeric Paint and gently brush it on the wax paper. I recommend making it lumpy and irregular. In this example, I am making a patch would.

You can make other different scar shapes of your choice, depending on the body location of the wound you want to simulate (and the shape of rubber sheets you previously prepared).

c) Immediately place your rubber sheet cut-outs on the elastomeric paint WHILE IT IS STILL WET. But if the paint is already dry when you decide to put on the rubber sheet cutout, don't despair, just add another fresh layer on the old one (or create a brand new similar shaped one) and place the rubber sheet there.
d) Once you have secured the rubber sheet on the elastomeric paint, add a little more in lumps on the sides of the rubber sheet. The paint should simulate you skin that was ripped off, so be sure it overlaps a bit over the sheet. It should look like this:

e) While the paint is still fresh, use again a new cotton bud and dip it on black paint (use very little) and then wipe gently on the sides of the paint where the "wound" starts and ends. This effect would simulate dried blood or rotten flesh as it being black or darker than the surrounding fresh wound.

03: Applying on Skin
a) When your scar prosthetic is dry, gently remove from the wax paper. Some of the wax paper may stick to the prosthetic but that is ok.
b) Once removed, clean the area of your skin with alcohol and tissue and prepare to apply Spirit Gum. This is available in Cinema Secrets Philippines. I've been hearing that there is another store in Quezon Ave that sells cheaper but i have not secured the info as of yet.

You may apply both on the prosthetic and on the skin you plan to stick it on. Wait a while after application before you stick it (around a minute) or if you cant wait, tap your fingers on the adhesive until you feel its sticky, this term is called "tacking". Once it feels sticky enough, apply on skin.

If you find it too inconvenient to buy Spirit Gum, then you can use chewed up bubblegum (chewed until there is no more sugar) and use it to stick it to your skin. The tip is courtesy of my friend Karumaru who used the concept on his Heath Ledge Joker scars. But the effect wont last as long as Spirit Gum, and sweat may remove its stickiness.

You may use this basic application to make more complicated prosthetics like these:

Notice how i colored the paint with black to mimic blackened wounds? This effect is best while the elastomeric paint is still wet and lumpy. Also, you can use the glue gun to apply traces of glue stick over the machinery and then follow it up with brushed elastomeric paint to simulate stretched bits of skin. Best to use cotton buds instead of an actual paint brush.

ADVANCED: Larger Prosthetic Molds

For larger prosthetics such as arms or faces, you may have to use body molds to have it a snug fit on your body part of choice. With this you will need Masking Tape. Lets start using an arm prosthetic as example.a) Apply strips of masking tape gently but firmly on your arm, on the area of choice, and make it two layers.
b) Gently remove the masking tape. It should have the curved shape of your arm.
c) Prepare a tubed object like a large PVC pipe or a used and empty 1.5 liter plastic softdrink bottle and wrap it with wax paper.
d) Place the masking tape mold on the wax paper tube, making sure some parts of the tape would stick on the wax paper, even for a bit.
e) With a paintbrush, apply elastomeric paint. It should look like this:

f) Let dry.

g) Prepare your rubber sheet machine parts. Base the parts on what you would expect from a human skeleton except that it has tubes and wiring and metal panels. Assemble the parts using super glue or glue gun and spray the whole thing in silver.
h) Glue gun the prosthetic machine part over the mold, but glue it in small points and not the whole edge. Then using again cotton buds, apply elastomeric paint on the machinery to mimic ripped or stretched skin. Apply glue gun on parts to mimic loose skin.

i) While wet, apply black paint on sides, again to mimic blackened blood or rotten flesh.

j) If you have an airbrush you can improve on the coloring, or have someone with airbrush skills do it for you.
k) Apply on skin using the same procedure with Spirit Gum or other effective non-toxic adhesives.l) Enjoy your new prosthetic costume for all costume occasions! More example pics of prosthetics below:

Last Advice: You can opt to make bigger and more complicated prosthetics but you must also put into account the biggest most important factor: GRAVITY. Spirit Gum strength can only carry so much, plus sweat can loosen its grip and you might find your prosthetics sliding in places you don't want them to.

Make sure you design large prosthetics that can be "hooked" on natural corners and points of your body. If you plan to make a large face prosthetic, make sure part of it goes over your skull to have a firmer grip. Shoulders and clavicles make good hook points, even elbows or armpits. Whatever you do, do not rely solely on the gripping power of skin adhesives. Best you either make small prosthetics, or make small sections that make up the big picture.